Gas
A gas is one of the four major states of matter, along with the solid, liquid, and plasma states. The gas state appears as a solid material is subjected to increasingly higher temperatures.
For example, consider an ice cube: as energy in the form of heat is added, the solid ice cube will first melt to become liquid water, which will then boil or evaporate to become water vapor, a gas. In some circumstances, a solid (such as "dry ice") can directly turn into a gas: this is called sublimation. If the gas is further heated, its atoms or molecules can become (wholly or partially) ionized, turning the gas into a plasma.
<<Here, give examples of some gases and summarize their biological and environmental roles and human applications.>>
Etymology
The word "gas" was apparently proposed by the 17th century Flemish chemist Jan Baptist van Helmont, as a phonetic spelling of his Dutch pronunciation of the Greek word "chaos," which was used since 1538 after Paracelsus for "air."
Physics
In a gas phase, the atoms or molecules constituting the matter basically move independently, (more freely than those in a solid or liquid) with no forces keeping them together or pushing them apart. Their only interactions are rare and random collisions. The particles move in random directions, at high speed. The range in speed is dependent on the temperature and defined by the Maxwell-Boltzmann distribution. Therefore, the gas phase is a completely disordered state. Following the second law of thermodynamics, gas particles will immediately diffuse to homogeneously fill any shape or volume of space that is made available to them.
The thermodynamic state of a gas is characterized by its volume, its temperature, which is determined by the average velocity or kinetic energy of the molecules, and its pressure, which measures the average force exerted by the molecules colliding against a surface. These variables are related by the fundamental gas laws, which state that the pressure in an ideal gas is proportional to its temperature and number of molecules, but inversely proportional to its volume.
Like liquids and plasmas, gases are flowing and free moving fluids: they have the ability to flow and do not tend to return to their former configuration after deformation, although they do have viscosity. Unlike liquids, however, unconstrained gases do not occupy a fixed volume, but expand to fill whatever space they can occupy. The kinetic energy per molecule in a gas is the second greatest of the states of matter (after plasma). Because of this high kinetic energy, gas atoms and molecules tend to bounce off of any containing surface and off one another, the more powerfully as the kinetic energy is increased. A common misconception is that the collisions of the molecules with each other is essential to explain gas pressure, but in fact their random velocities are sufficient to define that quantity. Mutual collisions are important only for establishing the Maxwell-Boltzmann distribution.
Gas particles are normally well separated, as opposed to liquid particles, which are in contact. A material particle (say a dust mote) in a gas moves in Brownian Motion. Since it is at the limit of (or beyond) current technology to observe individual gas particles (atoms or molecules), only theoretical calculations give suggestions as to how they move, but their motion is different from Brownian Motion, or the idea that finely divided particles in suspension undergo ceaseless movelent. The reason is that Brownian Motion involves a smooth drag due to the frictional force of many gas molecules, punctuated by violent collisions of an individual (or several) gas molecule(s) with the particle. The particle (generally consisting of millions or billions of atoms) thus moves in a jagged course, yet not so jagged as we would expect to find if we could examine an individual gas molecule.
Ideal gas
An ideal gas or perfect gas is a hypothetical gas consisting of a very large number of identical particles of zero volume, with no intermolecular forces, uniformly distributed in density. Additionally, the molecules, or atoms, have complete randomness of direction and velocity, and undergo perfectly elastic collisions with the walls of the container. Real gases do not exhibit these exact properties, although the approximation is often good enough to describe real gases. The approximation breaks down at high pressures and low temperatures, where the intermolecular forces play a greater role in determining the properties of the gas. There exist three basic types of ideal gas:
- the classical or Maxwell-Boltzmann ideal gas,
- the ideal quantum Bose gas, composed of bosons, and
- the ideal quantum Fermi gas, composed of fermions.
The classical ideal gas can be separated into two types: The classical thermodynamic ideal gas and the ideal quantum Boltzmann gas. Both are essentially the same, except that the classical thermodynamic ideal gas is based on classical thermodynamics alone, and certain thermodynamic parameters such as the entropy are only specified to within an undetermined additive constant. The ideal quantum Boltzmann gas overcomes this limitation by taking the limit of the quantum Bose gas and quantum Fermi gas in the limit of high temperature to specify these additive constants. The behavior of a quantum Boltzmann gas is the same as that of a classical ideal gas except for the specification of these constants. The results of the quantum Boltzmann gas are used in a number of cases including the Sackur-Tetrode equation for the entropy of an ideal gas and the Saha ionization equation for a weakly ionized plasma.
Classical thermodynamic ideal gas
The thermodynamic properties of an ideal gas can be described by two equations: The equation of state of a classical ideal gas is given by the ideal gas law.
The internal energy of an ideal gas is given by:
where:
The probability distribution of particles by velocity or energy is given by the Boltzmann distribution.
The ideal gas law is an extension of experimentally discovered gas laws. Real fluids at low density and high temperature, approximate the behavior of a classical ideal gas. However, at lower temperatures or higher density, a real fluid deviates strongly from the behavior of an ideal gas, particularly as it condenses from a gas into a liquid or solid.
Other basic laws
Boyle's law states that for a given mass of gas at a fixed temperature the product of pressure and volume is constant.
Charles' law or Guy-Lussac's law states that for a given mass of gas, if the pressure is kept constant, the volume increases linearly with the temperature.
Avogadro's law states that at constant temperature, pressure times volume will have the same value for all gases, provided they contain the same number of molecules. This is based on the assumption that two different gases at the same temperature must have the same average molecular kinetic energy, since they are each individually in equilibrium with the walls of the container.
Dalton's law of partial pressure states that at a fixed temperature, the pressure of a mixture of gases is equal to the sum of the pressures which would be exerted by each gas separately, if the other constituents were not there.
Ideal quantum gases
In the above mentioned Sackur-Tetrode equation, the best choice of the entropy constant was found to be proportional to the quantum thermal wavelength of a particle, and the point at which the argument of the logarithm becomes zero is roughly equal to the point at which the average distance between particles becomes equal to the thermal wavelength. In fact, quantum theory itself predicts the same thing. Any gas behaves as an ideal gas at high enough temperature and low enough density, but at the point where the Sackur-Tetrode equation begins to break down, the gas will begin to behave as a quantum gas, composed of either bosons or fermions.
Ideal Boltzmann gas
The ideal Boltzmann gas yields the same results as the classical thermodynamic gas, but makes the following identification for the undetermined constant Φ:
where Λ is the thermal de Broglie wavelength of the gas and g is the degeneracy of states.
Ideal Bose and Fermi gases
An ideal gas of bosons (e.g. a photon gas) will be governed by Bose-Einstein statistics and the distribution of energy will be in the form of a Bose-Einstein distribution. An ideal gas of fermions will be governed by Fermi-Dirac statistics and the distribution of energy will be in the form of a Fermi-Dirac distribution.
Biological and environmental roles
<<Briefly mention biological and environmental roles of some gases, such as oxygen, nitrogen, carbon dioxide, nitric oxide, and so forth.>>
Applications
<<Mention how people use some gases, such as methane, propane, hydrogen, noble gases, toxic gases, synthetic gases, and so forth. Some applications include: heating, lighting, energy transmission, anesthesia, aerosol sprays, and chemical warfare.>>
See also
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External links
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